1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to elevator systems which include means for determining when an elevator car is within a predetermined distance from a selected landing.
2. Description of the Prior Art
Certain functions in the control of an elevator car are initiated when the elevator car is within a predetermined distance from a selected landing, i.e., the landing at which the elevator car is preparing to stop. For example, the deceleration of an elevator car may be controlled in two modes, with the first mode being under the control of a distance dependent speed pattern generator which provides a signal for the drive motor control proportional to the square root of the distance to go to the floor, and with the second mode deriving the speed pattern signal directly from transducers disposed adjacent each floor. Control is transferred from the first to the second mode at a predetermined distance from the floor, such as 10 inches (25.4 cm.). U.S. Pat. Nos. 2,874,806; 3,207,265 and 3,747,710, which are assigned to the same assignee as the present application, describe such slowdown arrangements. Thus, it is necessary to know, very accurately, when the elevator car reaches the 10 inch (25.4 cm.) point when approaching the floor from either travel direction. This same 10 inch (25.4 cm.) signal may be used to initiate pre-opening of the doors of the elevator car and hoistway. Since two 10 inch (25.4 cm.) indicators are used, one for each direction, they may be vertically spaced about one-half inch (1.27 cm.) apart to provide a zone .+-. one-quarter inch (0.635 cm.) from floor level where neither indicator provides a signal, which indicates when the car is within one-quarter inch (0.635 cm.) of floor level. If the car moves outside this zone when standing at the floor with its doors open, one or the other of the 10 inch (25.4 cm.) indicators will initiate re-leveling.
In addition to the 10 inch (25.4 cm.) indicators, a second, completely independent indicator may be used which indicates when the car is within a predetermined zone adjacent the floor level, such as .+-. 2 inches (5.08 cm.) from floor level. This indicator represents a 4 inch (10.16 cm.) zone where the car is allowed to move below a very low speed with the doors open. If the car moves outside this zone with its doors open, or within the zone with the doors open but above a predetermined speed, certain protective actions are taken. Thus, it is desirable to know when the elevator car is within this .+-. 2 inch (5.08 cm.) zone relative to floor level.
Various control devices have been used for providing such control signals, such as mechanical switch/cam combinations, inductor relay/magnetic plate combinations, and photoelectric devices utilizing either photoconductive cells, such as photoresistors, or photovoltaic devices, such as phototransistors.
The cam operated mechanical switch is simple, but it is noisy and subject to mechanical wear and misalignment. The inductor relay devices are not subject to wear, but they are only useful for indicating approximate locations of the elevator car in the hoistway, such as for initiating slowdown points while the car is traveling at relatively high speeds several feet from the landing. In general inductor relays are not accurate enough to precisely indicate when the elevator car is within 10 inches of the landing, when the elevator car is within a .+-. one-quarter inch (0.635 cm.) zone from a landing, or when the elevator car is within a .+-. 2 inch (5.08 cm.) zone from the landing.
While the photoresistive devices provide the desired accuracy, they are not capable of operating at relatively high pulse rates, essential in light operated position indicators in order to reduce the incidence of false triggering caused by random light sources.
Photo devices, such as phototransistors, used in combination with light emitting diodes (LED) may be used to provide an excellent opto-electronic position detector for elevator systems, as they are capable of being pulsed in the kilohertz range, they are capable of providing the desired accuracy, and they have a long operating life. U.S. Pat. No. 3,743,056, which is assigned to the same assignee as the present application, discloses a new and improved opto-electronic position detector suitable for elevator systems, which is fail-safe, i.e., the failure of a circuit component will not cause false operation of the translating device or relay which is energized when two objects have a predetermined position relative to one another.
The position detector disclosed in U.S. Pat. No. 3,743,056 operates in the desired manner once the source of electromagnetic radiation, which will hereinafter be referred to as the LED, the target or reflector, and the photovoltaic device are properly aligned and mounted. It was found, however, that proper alignment and horizontal spacing of the components was critical and required considerable time and skill to properly mount these devices in the field. It was also found that the detector could be falsely actuated without the mirrored reflector due to a sensitivity of the circuitry to ambient light, coupled with "cross-talk" between the LED and phototransistor, and unwanted electrical coupling between other circuit components.